The present invention generally relates to an FM demodulating circuit employing quadrature detecting system and more particularly, to an FM demodulating circuit for a television receiver, video tape recorder or the like, having functions for subjecting a plurality of intermediate frequency signals with different frequencies, to FM demodulation.
As shown in FIG. 1, a quadrature detecting circuit is generally so arranged that an FM intermediate frequency signal produced from a limiter amplifier 1 at the last stage of an FM intermediate frequency amplifier, and a signal obtained by subjecting this FM intermediate frequency signal to phase shifting through a phase shift circuit 2, are respectively applied to a multiplier 3 so as to detect a phase difference between these two signals by the multiplier 3, the output of which is passed through a low-pass filter 4 connected thereto to obtain the FM demodulated signal. The limiter amplifier 1, multiplier 3 and low-pass filter 4 as described above are normally formed into an FM demodulating integrated circuit (IC) 6 together with an amplifier 5 coupled to the low-pass filter 4 for amplifying the FM demodulated signal produced from said low-pass filter 4. Meanwhile, for the phase shift circuit 2, there is generally employed a piezoelectric resonator, for example, a ceramic discriminator, which is connected to the FM demodulating integrated circuit 6 as a discrete part.
Incidentally, television broadcasting includes the NTSC system, PAL system, SECAM system, etc., and signal frequencies of the voice intermediate frequency signals differ according to such television broadcasting systems. Therefore, in television receivers, video tape recorders, etc. which are capable of changing over, for example, between the NTSC system and the PAL system, or between the PAL system and the SECAM system, etc. by change-over switches, it has conventionally been so arranged that FM demodulating circuits each including the FM demodulating integrated circuit 6 and phase shift circuit 2 are provided to correspond to the respective systems as described above for the demodulation of the voice intermediate frequencies, while such FM demodulating circuits are adapted to be changed over by switching circuits associated with said change-over switches.
In the known FM demodulating circuit as described so far, there have been such disadvantages that the plurality of the FM demodulating integrated circuits 6 are required, thus resulting in a complicated circuit construction, with an increase in the number of parts employed.